Impurities in CNTs appear in multiple forms and are often introduced during the synthesis of CNTs. In a typical manufacturing process called alcohol catalytic chemical vapor deposition (ACCVD), evaporated methanol or ethanol vapors come in contact with catalyst particles such as nickel or iron, embedded on magnesium oxide or silica as catalyst support, at high temperatures inside a furnace. At such conditions, ethanol or methanol molecules break down, and CNTs start growing around the catalyst. However, this process also results in the generation of amorphous carbon, which can be located randomly on the outer surfaces of CNTs. Amorphous carbon is the most common impurity and the hardest to remove, due to bonding on certain carbon atoms. Other types of impurities include catalyst residue such as iron, nickel, etc. and catalyst support materials such as magnesium oxide and silica. Apart from superficial contaminations, another type of defect, structural in nature, is caused by sp3 bonded carbon atoms replacing sp2 bonds in certain locations down the length of the CNT (FIG. 1). A reasonably large concentration of them can also be found at the ends of tubes.
A known technique useful for evaluating the quality of CNTs, i.e., the concentration of structural defects and amorphous carbon impurities included therein, is by measuring the intensity ratio of two characteristic Raman spectral peaks, called the G/D ratio. The G-band is a tangential shear mode of carbon atoms that corresponds to the stretching mode in the graphite plane. The D-band is a longitudinal optical (LO) phonon and is known as the disordered or defect mode, as it is a typical sign for defective graphitic structures in CNTs. When determining the quality level of a CNT sample via Raman spectroscopy, the absolute intensities of the G and D band peaks are not particularly relevant, and depend greatly on measurement conditions. Rather, the ratio of the intensity of the two peaks is the relevant measure. The comparison of the ratios of these two peaks' intensities gives a measure of the quality of the CNT samples. Generally, the G/D ratio is used to quantify the structural quality of carbon nanotubes. Thus, CNTs having a higher G/D indicate a lower amount of defects and a higher level of quality.
A G/D ratio is typically determined using a Raman spectroscopy technique. Any of various commercially available instruments may be used to measure the G and D band intensities and to calculate the G/D ratio. One example of such equipment is available from HORIBA Jobin Yvon Inc., Edison, N.J., under the model name LabRAM ARAMIS.
In a CNT sample, the G/D ratio is typically changed after purification, i.e., the G/D ratio of the purified CNTs is greater than the G/D ratio of the starting CNTs, indicating that the purified CNTs having fewer structural defects and/or carbonaceous impurities such as amorphous carbon.
Various methods of removing amorphous carbon or other carbonaceous impurities are known in the literature, including thermal oxidation, and various solution treatments. However, these methods tend to damage CNTs or cause loss of CNTs. A reported commercial method is the treatment of CNT with concentrated acid, such as nitric acid, followed by a slow heat treatment. Although this method has been proven to reduce both amorphous carbon and metallic content, it is unsafe, and a substantial amount of such contaminations can still remain on the surface. Furthermore, acid treatment is somewhat counterproductive, as it also introduces structural defects while removing superficial ones.
Therefore, there exists a need for an efficient and safe process for preparing purified CNTs; the method should efficiently remove carbonaceous impurities without damaging or destroying the CNTs.